Zirconia particles

ABSTRACT

Porous zirconia or zirconium-containing particles, methods of making such particles and methods of using such particles including modifications to the surface of the particles are described. The method comprises heating zirconia particles to provide a substantially homogeneously liquid melt, quenching the particles of melt to effect spinodal decomposition to provide quench particles of a silica rich phase and a zirconia rich phase, annealing the quenched particles to provide non porous solid particles of zirconia and silica and, leaching the silica from these particles to produce porous solid zirconia particles comprising a three dimensionally substantially continuous inter penetrating network of interconnected pores.

INTRODUCTION

The present invention relates, generally, to porous articles, and inparticular to porous zirconia or zirconium containing articles, tomethods of making such articles, and to methods of using such articles.One example of the porous articles are porous particles. Moreparticularly, the present invention relates to porous particlescontaining zirconia and other metallic oxides including silica incombination and to the manufacture and use of such particles. Even moreparticularly, the present invention relates to the use of particlescontaining zirconia and other metallic oxides including optionallycontaining silica, in separation applications, particularly inchromatographic applications. One particular aspect of the presentinvention relates to derivatisation processes whereby the surface of theporous zirconia or zirconium containing particles are modified and tothe use of such modified particles in chemical processes, particularlyin chromatographic applications.

Porous articles find use in certain applications because of theirproperties, such as for example, their high surface area per unitvolume. Such uses include use as supports for a wide variety of chemicalsubstances, such as catalyst supports and as chemical sorbents. Wherethe porosity and the pore size of the particles can be controlled, theporous particles also find particular use in chromatography applicationsand in chemical separation applications generally. Porous silica, oneexample of a porous particle, finds particular use in chromatographicapplications, such as High Performance Liquid Chromatography (HPLC).However, the use of porous silica is limited by the chemical reactivityof the particles since porous silica is susceptible to reactions inalkaline media and therefore is of only limited use in applicationswhich require resistance to alkaline attack or for operations conductedin alkaline media. Thus, there is a need for a porous material which isnot susceptible to alkaline attack and can be used in alkaline media.

Another example of porous articles are organic polymers which areparticularly useful in a wide variety of applications due to their poresize or to the pore sizes being readily controllable. However, at hightemperatures and in certain organic solvents, or when subjected tocertain mechanical stress, the organic polymers have limited strength,and can distort altering their pore sizes which in turn changes theseparation characteristics of the polymers and thus reduces theireffectiveness and usefulness in many applications. Disadvantages ofusing polymers are particularly prevalent in situations where thepolymer particles are mixed with liquids, since the low density of theorganic polymer particles, being similar to that of the liquids,prevents their ready separation from the liquid. In particular, lowdensity polymeric particles are difficult to handle in fluidised bedsdue to the similarities of the densities of the particles and of theliquids being treated in the fluidised bed. Thus, there is a need toprovide porous particles which retain their shape in a wide variety ofchemical and mechanical environments in order to prolong the usefulworking life of the particles and to increase the variety ofapplications in which the particles may be used. Additionally, there isa need to provide porous particles which can be readily separated fromthe liquids being treated by the particles on the basis of thedifference in densities of the particles and liquids.

In the past there has been a proposal to use porous zirconia particlesas the support phase for chromatography applications (Rigney, Webber andCarr, Journal of Chromatography 484 (1989) 273-291). However, thisproposal was not entirely successful due to the particles being unstablein some mechanical environments encountered in chromatographicapplications and due to the inability to modify the surface propertiesof the particles. Such disadvantages arose primarily from the methodused in making the particles. The present invention sets out to overcomethese and other weaknesses of the particles and of the previously usedmethod of making the particles.

Therefore, there is a need for porous particles which are resistant toalkaline attack, which are of improved strength and of high density,which can be used in a wide variety of chemical separation applicationsand which extend the applications in which such porous particles can beutilised by modifying the surface of the particles. It has now beendiscovered that it is possible to make porous is zirconia which canprovide improved resistance to alkaline attack, which is of goodstrength and has a relatively high density and which can be used indiverse chemical and mechanical environments in which hithertobefore ithas not been possible to use porous zirconia particles. The improvedproperties result at least in part from the method of making theparticles.

Porous Zirconia Particles

According to one aspect of the present invention there is providedporous zirconia particles or zirconium-containing particles in which theparticles comprise a substantially continuous three dimensionalinterpenetrating network of interconnected pores.

Typically, the pores of the particles are of substantially constantdiameter throughout their length. More typically, the pores havesubstantially constant diameter at the curves or bends of the pores, andat the intersection of the pores. However, it is to be noted that wheretwo or more pores intersect, the diameter of the pores may be changed toaccount for the individual pores not being exactly aligned with eachother.

Typically, the zirconia or zirconium-containing particles also comprisea further component. Typically, this component is a metal oxide, such asfor example silica. More typically, the particles of the presentinvention comprise a combination of zirconia and silica and canoptionally include zircon. Preferably, there is from 1 to 100% zirconiaand from 99 to 0% silica, more preferably 5-90% zirconia and 95-10%silica.

Typically, the size of the particles can be up to 200 μm or greater,preferably 5-100 μm, more preferably 5 to 80 μm and even more preferably10-70 μm.

Preferably, the porous zirconia of the present invention comprisesparticles having interconnected pores of up to about 2000 Å or greater,preferably between about 20 and 2000 Å in diameter, more preferablybetween 200 and 1500 Å in diameter, and even more preferably, pores ofbetween 500 and 1000 Å in diameter. However, it is to be noted thatpores of up to 5000 Å or even larger are possible with some of theparticles of the present invention depending on the size of theparticles. When the pore sizes become too large the effectiveness of theparticles in chemical separation applications reduces because thesurface area of the particles is reduced.

Typically, the surface area per unit mass of the particles can be up to100 m² /g, preferably 5 to 30 m² /g with a typical value being about 5m² /g.

Typically, the surfaces of the porous zirconia particles may bemodified, more typically, the outer surfaces of the particles orsurfaces of the pores closer to the outer surface of the particles. Moretypically, the surface modification of the particles involveshydroxylation of the surface to impart a greater amount of hydroxidegroups on the surface of the particles.

Even more typically, the surface modified or surface treated particlescan be further modified with other functional groups. The surfacemodified particles find particular usefulness in chromatographyapplications.

It will be understood that by use of the term "zirconia" in the presentspecification is meant zirconia-rich compositions such as those commonlyreferred to in the art as zirconia compositions or compositionscontaining a significant proportion of zirconia or zirconium, preferablyat least about 50% zirconia.

Crystallographic forms of Zirconia

Zirconia, which is also known as zirconium oxide (ZrO₂), may exist atroom temperature in any of three crystallographic forms; monoclinic,tetragonal or cubic. The monoclinic form is the most stable at roomtemperature, which is to say that this crystallographic form has thelowest bulk energy of all three forms at this temperature. Thetetragonal form is of higher energy than the monoclinic form and can bestabilised at room temperatures by the addition of dopants such as forexample, rare earth oxides including yttria, and also calcia andmagnesia. Preferably, the tetragonal form may be stabilised at roomtemperatures by the inclusion of yttria or other rare earth oxidesdepending upon the grain size of the crystallites of tetragonalzirconia, amongst other factors.

The transformation of zirconia from the tetragonal to monoclinic form isaccompanied by a 4% volume increase. Below a critical grain size, whichwill be dependent on a number of factors, including the nature of thematrix material in which the zirconia is embedded, the tetragonal formwill be metastable due to the fact that the increase in surface energywhich would accompany a 4% volume increase is greater than the reductionin the bulk energy on the transformation from the tetragonal tomonoclinic form.

The cubic form which is of the highest bulk energy is more unstable atroom temperatures than the other forms, and may be stabilised at thesetemperatures by the addition of dopants such as calcia and magnesia.

The crystallographic form of the zirconia present in particularparticles may be readily determined by any number of known methods,including X-ray diffraction.

It is to be noted that the porous zirconia particles orzirconia-containing particles of the present invention may take anycrystallographic form or combination of forms of the zirconia.

It has been found that the monoclinic form of the zirconia is thepreferred crystallographic form from which the porous zirconia particlesof the present invention can be composed and results in particles havingthe most desired properties for porous materials used in chemicalseparation applications. Therefore, it is preferable to use startingmaterials which produce porous monoclinic zirconia particles comprisingthe substantially continuous three dimentionally interpenetratingnetwork of interconnected pores.

Further it is to be noted that where it is desired to surface modify theporous zirconia particles, it is not at all critical that the porousparticles are composed of the monoclinic form of zirconia. The porousmonoclinic zirconia particles of the present invention exhibit improvedstrength and increased density when compared to conventional porousmaterials such as porous silica or porous organic polymers. Therefore,the porous monoclinic zirconia is particularly useful in applicationsfor the separation of chemicals and biochemicals, particularly usingchromatographic or biochromatographic techniques and other techniquessuch as batch procedures using stirred tanks, batch tanks, fluidisedbeds and the like.

The preparation of porous monoclinic zirconia having the requiredproperties requires careful control of the crystallographic structureand of the morphology of the zirconia. Therefore, another aspect of thepresent invention relates to a process for the production of porousmonoclinic zirconia.

Method of Making Porous Monoclinic Zirconia

According to the present invention there is provided a method for theproduction of porous monoclinic zirconia comprising the following stepsin sequence:

(a) heating a zirconia-silica composition to provide particles of saidcomposition in the form of a substantially homogeneous liquid melt;

(b) quenching said particles to effect spinodal decomposition of theliquid melt to provide quenched solid, non-porous particles comprising asilica-rich phase and a zirconia-rich phase, wherein the zirconia-richphase comprises zirconia substantially in tetragonal form;

(c) annealing said quenched particles to transform the tetragonal formof the zirconia-rich phase to the monoclinic form on cooling so as toprovide annealed particles comprising a continuous monocliniczircornia-rich phase and a continuous silica-rich phase;

(d) leaching said silica-rich phase from the annealed particles toprovide porous monoclinic zirconia comprising a three dimensionallysubstantially continuous interpenetrating network of interconnectedpores.

In the process for producing porous monoclinic zirconia particlesaccording to the present invention, a zirconia-silica composition isused as the starting composition, which zirconia-silica composition isheated to form the homogeneous liquid melt which undergoes phaseseparation on cooling to form one phase of zirconia and another ofsilica.

Typically, in step (c) a third phase is formed. This third phase istypically zircon. More typically, the zircon phase is not leached awaywhen the silica phase is being leached. Even more typically, the porousparticles contain zircon in addition to the zirconia.

The zirconia-silica composition used in step (a) may be either anadmixture of zirconia or a zirconia-containing material and silica or asilica-containing material or may be a compound containing bothzirconium or zirconia and silica or combination thereof. Additionally,compositions or compounds which decomposes to provide a homogeneousliquid melt of zirconia and silica may be used.

Preferably, the starting material used in this method of the presentinvention is commercially available zircon. More preferably the zirconundergoes a pretreatment such as for example sieving or similar to suitthe end uses to which the porous particles are to be put. Typically, thecommercial zircon is screened to remove coarse particles greater than100 μm.

Typically, the zirconia:silica volume ratio in the quenched particles isabout 1:1. This ratio provides porous monoclinic zirconia after leachingof a particularly uniform structure and of substantially even porosity.The molar volumes of both silica and zirconia are very similar to eachother and hence it is desirous to select a zirconia-silica compositionwherein the molar ratio of zirconia to silica is about 1:1, in order toachieve a volume ratio of about 1:1. Where the zirconia-silicacomposition used in this form of the method of the present invention isa composition which decomposes to a homogeneous liquid melt of zirconiaand silica the composition should decompose to give a mixture ofzirconia and silica in a molar ratio of about 1:1. However, it is to benoted that the ratio of zirconia to silica can be altered according tothe final properties required of the porous particles since the leachingof the silica is responsible for the productions of pores in thezirconia particles and hence the amount of silica originally present inthe starting zirconia-silica composition determines at least in part theamount of the pores present in the particles.

Typically, zircon is a particularly preferred form of the startingzirconia-silica composition. Zircon has the molecular formula ZrSiO₄ anddecomposes to a 1:1 molar mixture of zirconia (ZrO₂) and silica (SiO₂).

Typically, admixtures of zirconia and silica or materials containingthese compounds may also be conveniently used to provide a volume ratioof about 1:1. However, it is to be noted that any ratio may be used.Admixtures may also be used to provide varying volume ratios, therebyallowing the degree or amount of porosity to be controlled in the porouszirconia.

Typically, the pore size of a single particle is substantially constant.However, the pore sizes may vary between particles. A typical pore sizedistribution can be from 0.01 to 0.2 μm for a particle size range offrom 40 to 80 μm.

Typically, the zirconia-silica composition may conveniently be providedin the form of a powder or particles. The size of the particles will bedetermined by a number of factors. The zirconia-silica particles aredesirably of a size conveniently adapted for the end use of the porouszirconia in the desired application, such as, for example, use aschromatographic powders and the like. The zirconia-silica particles arepreferably sufficiently small so as to be able to form a homogeneousliquid melt. This size will be determined by such things as the heatingrate, heating time and thermal conductivity of the zirconia-silicaparticles amongst other factors. The zirconia-silica particles shouldalso be sufficiently small so as to allow the homogeneous liquid melt tobe quenched at a rate which allows spinodal decomposition of the liquidmelt as will be discussed in more detail later in this specification.This size will be determined by the required cooling rate which itselfis dependent on the composition of the zirconia-silica composition, thetemperature of the quenching medium, the efficiency of heat transferfrom the particles to the quenching medium and the thermal conductivityof the particles among other factors.

Typically, the zirconia-silica composition should ideally be heated toprovide a homogeneous liquid melt. The temperature necessary will bedependent on the zirconia-silica composition. For example, thezirconia-silica composition which is equivalent to that of zircon formsa liquid at temperatures in excess of about 2400° C. Typically, thezirconia-silica composition would be heated to temperatures well abovetheir respective melting temperatures so that the time for forming thehomogeneous liquid melt is reduced.

Typically, the zirconia-silica composition may be heated in anyconvenient heating apparatus available to the skilled artisan, whichheating apparatus is capable of producing temperatures sufficiently highto melt the zirconia-silica composition. It has been found particularlypreferable to utilise a plasma arc torch or reactor to heat thezirconia-silica composition. When using a plasma arc reactor thezirconia-silica composition is preferably in the form of particlescomprising an intimate mixture of zirconia and silica or in the form ofa composition which will decompose to form an intimate mixture of silicaand zirconia. Zircon may be conveniently used as one example of thezirconia-silica composition in a plasma arc torch. The particularlypreferred particles of zircon for use in a plasma arc torch have aparticle size in the range of from 5 μm to 100 μm in size. Morepreferably, the zircon particles are in the range of 10 to 55 μm.Typically, the zircon particles are elongated, or acicular and onheating the particles first disassociate and then melt to form ahomogeneous liquid melt.

Typically, the use of smaller zirconia-silica particles allows the useof lower temperatures for heating, such as for example, passing throughan oxyacetylene or oxyhydrogen flame. Typical particle sizes useful withflame spraying are in the range of from 3 to 15 μm.

Typically, the particles of the homogeneous liquid melt are quenched ata cooling rate sufficient to prevent nucleation and growth of zirconiaspherulites and to allow spinodal decomposition of the liquid melt intozirconia-rich and silica-rich phases. The spinodal decomposition of thehomogeneous liquid melt gives an extremely fine microstructure ofzirconia-rich and silica-rich interpenetrating networks which exhibituniform periodicity and three dimensional continuity.

Typically, the quenched solid, non-porous particles have wave lengths ofapproximately 100 Å between the different phases. By "wave length" inthe present specification is meant the average distance between onephase and the next.

By the term "uniform periodicity" is meant the wave length issubstantially uniform.

By the term "three dimensional continuity" is meant that each phaseforms an interconnected three dimensional network.

Typically, the spheroidal particles formed from the homogeneous liquidmelt are quenched in a water bath. However, other liquid quenching mediamay be used. Liquid quenching media are preferred due to the high heattransfer rates which can be achieved between the particles and the;liquid.

Typically, quenching will provide a cooling rate of the order of about10⁵ to 10⁷ °C. sec⁻¹. However, other quenching rates are possible.

It will be understood by those skilled in the art that by the term"zirconia-rich" is meant a phase containing a higher percentage ofzirconia than contained in the original homogenous liquid melt of thezirconia-silica composition.

It will be understood by those skilled in the art that by the term"silica-rich" is meant a phase containing a higher percentage of silicathan in the original homogeneous liquid melt of the zirconia-silicacomposition.

Typically, the quenched particles comprise both a zirconia-rich and asilica-rich phase. The zirconia-rich phase will be substantially in thetetragonal form which is metastable and on leaching of the silica-richphase therefrom will transform to the stable monoclinic form. Thistransformation is accompanied by a 4% volume increase which generallyleads to the disintegration of the zirconia-rich network. Therefore, ifit were not for the annealing stage prior to the leaching stage it wouldnot be possible to produce porous particles having the characteristicsand properties possessed by the particles of the present invention.

The quenched particles are therefore annealed to coarsen thezirconia-rich phase. Typically, the annealing takes place below thetemperature at which substantial recombination of zirconia and silicaoccurs at an appreciable rate. This temperature will be dependent on thecomposition of the zircona-rich phase and be readily determinable bysimple experimentation by the skilled artisan. However, it is to benoted that it is preferable for some recombination of the silica andzirconia to take place to form zircon of a similar structure to enhancethe strength of the porous particles produced by leaching the uncombinedsilica therefrom.

Preferably, the annealing takes place at a temperature sufficient forthe coarsening of the zirconia-rich phase at a rate convenient formanufacture, which is to say at a rate which is fast enough to beaccomplished on a reasonable time scale, but not so fast as to renderthe coarsening uncontrollable. Preferably, the annealing takes place ata temperature in the range of from about 1000° C. to 1400° C.,preferably 1200° C. to 1400° C. and is achieved over a period of up to 5hours, preferably from about 1 to 5 hours depending on the particle sizeof the particles.

Typically, the degree of coarsening of the zirconia-rich matrix aids indetermining the pore size in the porous zirconia. The longer or moreextensive the coarsening the larger the pore size in the porouszirconia. Typically, the coarsening of the zirconia-rich phase occurs bydiffusion of zirconia and silica. During coarsening, the grain size ofthe zirconia crystallites increases. Typically, the zirconia is of agrain size sufficient to allow the zirconia to transform from thetetragonal form to the monoclinic form on cooling to ambienttemperature. The critical grain size is dependent on the composition ofthe zirconia-rich phase. The temperature of the transformation from thetetragonal to monoclinic form is dependent on the composition of thezirconia-rich phase and the grain size of the zirconia crystallites.However, it is to be noted that it is preferable to have some silica orzircon present in the zirconia phase before leaching to prevent collapseof the substantially pure zirconia particles.

The transformation temperature of dissociated zircon that has beenquenched and subsequently annealed is typically about 720° C. After thequenched particles have been annealed it is preferred that the particlesare cooled slowly through the transformation temperature so as to avoiddisintegration of the particles so that particles having improvedstrength can be obtained.

It is preferred that the zirconia-rich phase is not coarsened to such anextent that the transformation of the tetragonal form to the monoclinicform, with its accompanying 4% volume increase, leads to shattering ofthe annealed particles on cooling. Above a critical size, determined bythe composition of the silica-rich phase among other factors, thezirconia-rich phase on transformation introduces strains into thesilica-rich phase which can cause it to fail on cooling.

The annealed particles are then leached to remove the silica-rich phase.Alkali or hydrofluoric acid may be used to leach the silica-rich phase.Preferably, the annealed particles are leached with alkali, morepreferably with sodium hydroxide. Typically, the rate of leaching isincreased with increased temperature. More preferably, the annealedparticles are leached with sodium hydroxide at a temperature of about160° C.

Method of Making Porous Tetragonal Zirconia

According to another aspect of the present invention there is provided aprocess for the production of porous tetragonal zirconia comprising thefollowing steps in sequence:

(a) heating a zirconia-silica composition to provide particles of saidcomposition in the form of a substantially homogeneous liquid melt;

(b) quenching said particles to effect spinodal decomposition of theliquid melt to provide quenched particles comprising a silica-rich phaseand a zirconia-rich phase;

(c) annealing said quenched particles to coarsen the zirconia-rich phaseso that the desired pore diameter can be obtained after step (d); and

(d) leaching said silica-rich phase from the annealed particles toprovide porous tetragonal zirconia comprising a three dimensionallycontinuous interpenetrating network of interconnected pores.

Preferably, the tetragonal zirconia is stabilised by the addition ofdopants, such as for example, rare earth oxides including yttria, calciaor magnesia or combinations thereof. More preferably, yttria is used asthe dopant. In order to produce porous tetragonal zirconia it ispreferred that the dopants are intimately incorporated into thezirconia-silica composition in the initial heating step. This enables ahomogeneous liquid melt to be readily formed on heating.

Typically, the zirconia-silica composition further comprises a dopant.Typically, the dopant is a rare earth oxide, preferably yttria.Typically, the dopant exists as particles in the zirconia-silicacomposition. More typically, two phases are formed in step (b), thedopant being preferably incorporated into the zirconia-rich phase.

The process for producing porous tetragonal zirconia as hereinabovedescribed may be carried out according to the parameters described withreference to the process for producing porous monoclinic zirconia exceptfor the annealing step. The annealing step in the process for producingporous tetragonal zirconia preferably produces grains of tetragonalzirconia which are stabilised by the presence of the dopant with respectto transformation to the monoclinic form on cooling to ambienttemperature and throughout the leaching process. The stability of thegrains of tetragonal zirconia is dependent on the composition and amountof dopants in the zirconia-rich phase among other factors.

Porous tetragonal zirconia is particularly strong and can provideparticles which are particularly useful due to their strong and robustnature.

Method of Making Cubic zirconia

According to another aspect of the present invention there is provided aprocess for the production of porous cubic zirconia comprising thefollowing steps in sequence:

(a) heating a zirconia-silica composition to provide particles of saidcomposition in the form of a substantially homogeneous liquid melt;

(b) quenching said particles to effect spinodal decomposition of theliquid melt to provide quenched solid, non-porous particles comprising asilica-rich phase and a zirconia-rich phase;

(c) annealing said quenched particles to coarsen the zirconia-rich phaseso that the desired pore diameter can be obtained after step (d); and

(d) leaching said silica-rich phase from the annealed particles toprovide porous cubic zirconia comprising a three dimensionallycontinuous interpenetrating network of pores.

Preferably, the cubic zirconia is stabilised by the addition of dopants,such as, for example, calcia, magnesia and the like. Typically, thedopant must be present in sufficient quantities to stabilise the cubicform of the zirconia. In order to produce porous cubic zirconia it ispreferred that the dopants are intimately incorporated into thezirconia-silica composition. This enables a. homogeneous liquid melt tobe readily formed on heating.

The process for producing porous cubic zirconia as hereinabove describedmay be carried out according to the parameters described with referenceto the process for producing porous monoclinic zirconia except for theannealing step. The annealing step in the process for producing porouscubic zirconia preferably produces grains of cubic zirconia which arestabilised with respect to transformation to either the tetragonal orsubsequently monoclinic form on cooling to ambient temperature andthroughout the leaching process. It is to be noted that while theannealing step is not necessary to stabilise the zirconia particles, itinfluences the pore size of the particles. The stability of the grainsof cubic zirconia is dependent on the composition and amount of dopantsin the zirconia-rich phase as well as the grain size among otherfactors.

Derivatised Porous Zirconia

It is a further object of the present invention to provide derivatisedporous zirconia particles with enhanced stability under alkalineconditions, with enhanced strength in a wider variety of harshenvironments so that such particles can be used in a wider variety ofchemical separation applications, and with a wide variety of functionalgroups on the surface of the particles so that the porous particles canbe used in a wide variety of chemical separation processes, includingchromatographic applications.

According to the present invention there is provided porous zirconiaparticles having functional molecules attached to the surface of theparticles via a silane group reacting with surface hydroxyls on theparticle surface of the zirconia particles.

In a further form of the present invention there is provided porouszirconia particles having a shell of organic polymer around orsurrounding the zirconia particles wherein the polymeric shell iscross-linked and attached to the hydroxyl groups on the surface viasilanes.

The present invention also relates to a method of preparing derivatisedporous zirconia particles by first treating the particles via ahydrothermal process to increase the hydroxyl group density on theparticle surface, and then reacting the hydroxyl surface group with asilane.

Silane is a term recognised in the art relating to silicon hydrides andincludes disilanes as well as trisilanes. Other chemical groups may becoupled to the silane molecules.

The hydrothermal treatment as practised in accordance with the presentinvention is used to reintroduce hydroxyl groups to the surface of azirconia particle and to provide a high and uniform hydroxyl groupdensity on the surface. The hydrothermal treatment is performed attemperatures between 100 and 300° C., typically 150° C. and elevatedpressures. The pressure inside the autoclave is due to vapour pressureof water and is a function of the temperature of the autoclave.

A high and uniform hydroxyl group density is a requirement for a highligand density during subsequent modification of the surface propertiesof the zirconia particle. The quality of the modification and thereforethe effectiveness of the hydrothermal treatment is measured indirectlyby determination of the uncovered zirconia surface area. Theeffectiveness of the hydrothermal treatment is dependent on the durationof the treatment and the temperature (pressure) involved. The higher thetemperature the faster the kinetics. As an example, using a temperatureof 150° C. the optimum reaction time for the hydrothermal treatment ofthe zirconia particles is 6 hours. However, any temperature, time,pressure combination within the limits of each of these parameters canbe used in the hydrothermal treatment to hydroxylate the surface of theparticles.

In a further embodiment-the invention provides a method of preparingderivatised porous zirconia particles with a polymeric shell on thesurface by adsorbing a monomeric material onto the surface and thenpolymerising the monomeric material to form the polymeric shell.

The porous zirconia of the present invention may be derivatised by theattachment of organic molecules to the surface of the porous zirconia.Such organic molecules which may be attached to the surface of theporous zirconia include affinity dyes, hydrophobic and hydrophilicsurfaces and the like, one example of which are the silanes. Typically,the surface of the porous zirconia particles is activated with asubstituted silane onto which the organic molecule is bound. Thederivatisation of the porous zirconia provides an increased range ofseparation applications for which the porous zirconia may be used andthus extends the application of the present invention.

It is to be noted that the porous zirconia or zirconia-containing orzirconium containing particles, optionally containing other metallicoxides, such as rare earth oxides and including silica, may be modifiedby attaching any suitable, desirable or convenient chemical groups ormolecules onto the surface depending on the properties desired of theparticles and the applications in which the particles are to be used.

Examples of chemical groups or molecules that can be attached to theparticles, including the following: Hydrophobic ligands in the form ofalkyl chains, aromates or cyano groups, hydrophilic ligands likepolyols, carbohydrates, polyethers or polyesters, anionic and cationicion exchangers over a wide range of ionic strength, peptides, proteins,enzymes, metal chelates and molecules forming specific interactions withbiological active substances, lipids, DNA, RNA, dyes oligonucleotides,and the like. It is to be noted that the foregoing list is notexhaustive but rather is merely illustrative, as would be apparent tothe skilled worker.

It will be evident to those skilled in the art that the list is notexclusive.

Another embodiment of the present invention relates to the use of porousparticles in a process of chromatographically separating proteaceousmolecules.

The invention will now be described by way of example with reference tothe accompanying drawings and the following non-limiting examples inwhich:

IN THE DRAWINGS

FIG. 1 is a high magnification photo-micrograph of a caustic leachedannealed particle of zirconia showing the interconnected pore structurein accordance with the present invention.

FIG. 2 is a modification of zirconia with Cibacron Blue F3GA.

FIG. 3 is a modification of zirconia with octadecylsilane.

FIG. 4 is a modification and crosslinking of zirconia withcarbohydrates.

FIG. 5 is a modification of zirconia with aminoacetic acid.

FIG. 6 is a modification of zirconia with a protein such asConcanavalin-A, pepsin, papain, trypsin, chymotrypsin.

FIG. 7 and FIG. 8 are flow charts of the steps of typical examples ofmaking the particles in accordance with the present invention showingtypical processing conditions. It is to be noted that not all particlesare made by a method involving all of the steps shown.

EXAMPLE 1 Manufacture of Monoclinic Zirconia Particles

Zircon flour (supplied by Commercial Minerals) was first sifted toremove particles coarser than ˜50 μm and then flame sprayed in order toimprove the flow characteristics of the powder such as by rounding orsmoothing of the particles' surfaces. The flame spraying was carried outin a Metco Type 6P-II Thermospray Gun flame spraying torch, specificallydesigned for the flame spraying of ceramic and metallic powders. Thepowders were injected into an oxy-acetylene flame with the assistance ofa Metco Type 4PM Powder feed unit. The oxy-acetylene flame beingslightly oxidising, this resulted in the finer (<˜10 μm diameter)particles being spheroidised. It is to be noted that the flametemperature was not sufficiently high to melt and spheroidise the largerparticles. This step was found to be necessary in some circumstances asit was found to be difficult to introduce the untreated powder directlyinto the plasma torch on occasions. The flame sprayed powder wascollected in distilled water. After the powder had been dried, it wasthen plasma sprayed, and again collected in distilled water. The powderwas plasma sprayed in a DC plasma torch (Plasmadyne SG-100, 40 kWsubsonic plasma torch) which produced a 36 kilowatt Ar/He plasma jet.The powder injection was into the plasma tail flame. This resulted inthe spheroidisation of all the zircon particles.

X-ray diffraction analysis of the powder on a Rigaku Geigerflex systemequipped with a wide angle goniometer was used to determine thecrystalline phases present. The X-ray diffraction analysis revealed thatthe zirconia was present in the tetragonal form of zirconia. The silicaproduced from the dissociation of the zircon during the plasma treatmentwas amorphous (glassy).

The powder was then heat treated in a Rapid High Temperature Furnace(Kanthal) at 1400° C. for 2 hours in order to coarsen the spinodalstructure to such an extent that the tetragonal zirconia would transformto the thermodynamically stable monoclinic form on cooling. X-raydiffraction was used to confirm this. In order to prevent reduction inmechanical strength of the particles due to the phase transformation avery slow cooling rate was used through 720° C., the transformationtemperature.

The silica was removed from the powder by leaching with a 60% aqueoussolution of NaOH at 160° C. in a nickel crucible.

The powders were then examined under the SEM to check the morphology ofthe particles and also to obtain a particle size distribution.

Powder sizing--The spheroidised powders were mixed with water and thenseparated into narrow size ranges using a Warman Cyclosizing-apparatus.This consists of a series of five cyclones, the size ranges trapped ineach cyclone depending on the operating parameters as well as theparticle size and shape.

Particle size analysis--Particle size analysis was carried out by thedirect measurement of SEM photographs of the particles using a ZeissParticle Size Analyser TGZ-3.

Scanning Electron Microscopy (SEM)--A JEOL JSM-840 scanning electronmicroscope equipped with energy dispersive X-ray analysis facilities wasused to examine the particles. Both secondary electron images and backscattered electron images were obtained. The latter gave atomic numbercontrast.

A typical pore structure of a zirconia particle made by the above methodis shown in FIG. 1.

EXAMPLE 2

Two types of ceramic particles based on zirconia were used in thecomparison of this example. One (referred to as PDZ later in thisexample) was prepared in the Department of Materials Engineering atMonash University in accordance with the methods of the presentinvention. These particles had an average size of 7 μm, a pore size of1000 Å and a specific surface area of between 1.0 and 4.2 m² /g(measured by BET). The other particles were provided by the 3M company,St. Paul, Minn. 55144, USA, (Batch-No. 90 588 P15) and were made inaccordance with a different process (i.e. the precipitation process) tothat of the present invention for use as a comparison to the PDZmaterials. These particles had an average particle size of 15 μm a poresize of 160A and a surface area of 32 m² /g.

Determination of the Surface Area by Adsorption of Phosphate

Phosphate anions are known to bind strongly to zirconia surfaces.Therefore, the amount of bound phosphate ions on the support particlescan be used to determine the surface area of this support or aftermodification to determine the remaining uncovered surface of theparticles.

These measurements were used as an alternative to elemental analysis todetermine the success of the modification process of the presentinvention.

For this purpose a phosphate solution of known phosphate concentrationwas prepared. A part of this solution was stored as a standard solutionfor the determination of the concentration. To the other part zirconiaparticles were added and the suspension was shaken over night. Then thesolid parts were removed by filtration and the phosphate concentrationof the supernatant was measured.

EXAMPLE 3 Hydrothermal treatment to increase the hydroxyl group densityon the zirconia surface

To increase the hydroxyl group density on zirconia surfaces thehydrothermal treatment as previously described was able to achieve ahigher amount of-reactive hydroxyl groups for the modification.

The zirconia particles were treated in an autoclave in a water steamatmosphere at 150° C. for different times reaching from 1 to 16 hours.After the treatment the particles were modified with a C₁₈ -silane andthe uncovered zirconia surface was determined by the adsorption ofphosphate ions. One possible set of conditions to achieve optimalresults was 6 hours at 150° C.

EXAMPLE 4 Molybdenum Blue Method

Orthophosphate and molybdate ions form an acidic solution ofmolybdophosphoric acid, which can be selectively reduced by hydrazinesulphate to form molybdenum blue, a compound of uncertain composition.This complex can be measured photometrically at its absorption maximumat 820-830 mn.

Procedure: The concentration of the sample should be smaller than 4 mgphosphorus per litre. 50 μl of sample at neutral pH was mixed with 10 μlmolybdate solution and 4 μl hydrazine sulphate solution and diluted to100 μl. The mixture was heated in a boiling water bath for 10 minutesand then cooled rapidly. The volume was adjusted and the absorption wasmeasured at 690 nm. The absorption of the sample was measured in amicrotiter plate together with different dilutions of the standardphosphate solution as calibration.

Table I shows the results of the phosphate adsorption on differentmodified and non-modified support materials. From the values obtained inTable I it can be readily seen that the amount of phosphate adsorbed onPDZ zirconia was considerable less than the amount adsorbed on thezirconia for both unmodified and modified forms of the respectivezirconia. This indicates that the hydrothermal treatment of the zirconiasignificantly improves the ligand density during the modification step.Furthermore, the performance of the zirconia particles made inaccordance with the present invention performed significantly betterthan the 3M derived particles due, it is thought, to the differentstructure of the pores of the particles made in accordance with thepresent invention which structure only clearly distinguishes the PDZparticles of the present invention.

Modification of the supports

Two principally different methods to modify the surface of a sorbentparticle are available. The first approach is to use a silane which willreact with a hydroxyl group present on the support surface. This willlead to a monomeric modification. The other possibility to introduce adesired interactive surface is to cover the surface of the particle witha polymer coating. The polymeric coating can but does not have to becovalently attached to the surface.

EXAMPLE 5 Modification with Mercantosilane and Cibacron Blue F3GA

To study the possibility to derivatise the zirconia particles a CibacronBlue modification was chosen because it is easy to see whether themodification was successful or not by observation of the intensity ofcolour of the final product as shown in FIG. 2.

The modification with Cibacron Blue was performed in three steps. Firsta hydrothermal treatment, as described above (150° C., 6 hours) wasperformed to insure a high and uniform hydroxyl group distribution onthe zirconia surface. Second the zirconia particles were activated with3-mercaptopropyl-trimethoxysilane and then modified with Cibacron BlueF3GA. To couple the silane to the surface of the particles, theparticles were suspended in nitric acid at a pH of 3.5. The silane wasadded and the suspension was shaken at 90° C. for three hours. Thebinding of the triazine dye was performed at 60° C. in sodium carbonatebuffer at a pH of 8.0 containing 0.5M NaCl overnight.

The amount of silane necessary to modify the particles was calculated bythe product of the specific surface area, the amount of zirconia, thehydroxyl group density (about 8 μmol/m²) and the molecular weight of thesilane. Because of steric reasons only half of the hydroxyl groups areaccessible for the silane. Therefore, using 8 μmol/m² as hydroxyl groupdensity results in a twofold excess of silane. A higher amount of silaneshould be avoided because of the tendency of the trimethoxy group topolymerise which may fill up some pores then rendering the particlesless useful in subsequent applications.

Immobilising the dye is not limited by the number of reactive sites onthe particles' surface but by the size of the molecule. The maximumamount of dye able to bind to the support is about 1 μmol/m². Again, atwofold excess was used for the reaction. After the reaction wascompleted the supports were washed with water and 2-propanol.

EXAMPLE 6 Modification with Octadecyldimethyl-chlorosilane

First a hydrothermal treatment, as described above (150° C., 6 hours)was performed to insure a high and uniform hydroxyl group distributionon the zirconia surface. The ceramic support materials were modifiedwith octadecyldimethyl-chlorosilane (ODS) in order to achieveRP-sorbents. The modification was performed in anhydrous toluene, usingimidazole as a catalyst. The toluene was stored over sodium metal andfreshly distilled before use. To remove physically adsorbed water fromthe surface of the particles the particles were suspended in thesolvent, the imidazole and the silane were added and the mixture treatedan an ultrasonic bath for five minutes and then heated under reflux forsix hours. The silane was added in an eightfold excess assuming that themaximum ligand density is about 4 μmol/m² as shown in FIG. 3.

To prevent a grinding of the particles the use of a magnetic stirrer wasavoided. After the reaction was finished, the sorbent material waswashed with toluene, 2-propanol and water.

EXAMPLE 7 Modification with Polybutadiene

Another method of producing a reversed phase material in accordance withthe present invention is to attach a polymeric layer onto the surface.Depending on the amount of polymer desired to bind on the surfacedifferent methods to prepare these support materials are available. Thepolymeric layer should not be too thick otherwise it will fill up thepores and decrease the surface area to a very high degree thus reducingthe effectiveness of the particles. Pretreatment of the particles toincrease the hydroxyl group surface concentration was not necessary inall cases for the coating with polybutadiene, but could be used ifdesired or required.

The particles were modified using two different amounts ofprepolymerised polybutadiene (PBD) resulting in supports with differentthickness of the polymeric layer. For the low carbon loading, the amountof PBD was calculated to be 0.5 mg/m². The PBD and the dicumylperoxide(DCP) were dissolved in dry pentane and the dried zirconia particlesmade in accordance with the method of the present invention were added.The pentane was removed under vacuum and the coated particles wereheated to 60° C. under vacuum for 12 hours. The final step was a heattreatment at 200° C. under nitrogen atmosphere for 4 hours to crosslinkthe coating. To effect modification with the polybutadiene it ispreferred that the particles have a large pore size.

EXAMPLE 8 Modification with Aminosilane and Carbohydrate

The purpose of this Example was to produce a hydrophilic bonded phasewhich would be easy to derivatise and which would have a high pHstability. Glucose and Maltose were coupled to aminopropyl derivatisedPDZ-powder. First a hydrothermal treatment, as described above (150° C.,6 hours) was performed to insure a high and uniform hydroxyl groupdistribution on the zirconia surface. To 1 g of zirconia (driedovernight under vacuum at 180° C.) suspended in 50 ml anhydrous toluenean amount of 3-aminopropyltri-methoxysilane was added corresponding to atwofold excess compared to the accessible hydroxyl group density on thezirconia surface (as described in Example 5 for the modification with3-mercaptopropyl triethoxysilane). The reaction was performed bytreating the suspension under reflux for six hours. After completion theparticles were extensively washed with toluene, 2-propanol, 10 mM HCland water.

Glucose or maltose was coupled to the aminopropyl zirconia in a 50 mMsodium carbonate buffer pH 6.8. An estimated 10 times excess of glucoseor maltose was used for the coupling, which was performed by shaking thesuspension at 60° C. overnight. An equimolar amount compared to theamount of carbohydrate of sodiumcyanoborohydride was included to reducethe Schiff's base that is formed. After the reaction was completed theparticles were washed and suspended in acetone to crosslink differentamounts of butadiene diepoxide reaching from 10 to 100 μl per gram ofparticles have been used. The crosslinking reaction was performed fortwo hours at ambient temperature with borontrifluoride diethyletherateas catalyst.

Any remaining epoxide rings were opened either by an acid treatment orby deactivation with ethanolamine. The derivatised sorbents were eitherused without any further treatment or modified with Cibacron Blue F3GA.For the modification the particles were suspended in 100 mM Sodiumcarbonate buffer pH 9.5 containing 0.5M NaCl and an excessive amount ofCibacron Blue was dissolved. The reaction was performed at 60° C.overnight, after which the particles were washed with water and2-propanol to give the results in FIG. 4. The amount of coupledaminosilane was determined by elemental analysis and coupled glucose viathe difference between noncoupled glucose in the supernatant and glucosein the "coupling solution". The result indicates that 97% of the aminogroups are derivatised by a glucose unit. This is supported by the factthat zirconia modified via aminosilane and glucose does not give anycolour reaction with picryl sulphonic acid, a reagent detecting aminogroup.

Modification with 3-glycidoxypropyl-trimethoxysilane (Glymo)

Two different modification procedures were used; modification atanhydrous conditions and modifications in aqueous solution at acidicconditions. Under anhydrous conditions the silane will bindmonomerically to the zirconia while at acidic conditions a polymericlayer will be formed.

EXAMPLE 9 Modification under Anhydrous Conditions

Two grams of zirconia were rehydroxylated with hydrothermal treatment asdescribed in Example 3 at 150° C. for six hours, after which theparticles were dried under vacuum at 180° C. overnight. The particleswere suspended in 50 ml anhydrous toluene. 17 mg silane and 15 mgimidazole as a catalyst were added and the suspension was treated underreflux for six hours. The modified particles were washed with toluene,2-propanol and water.

EXAMPLE 10 Modification under Aqueous Conditions at Acidic pH

The zirconia particles were rehydroxylated as described under Example 3.Two grams zirconia were suspended in 20 ml of a 10% solution of Glymo inwater adjusted at pH 3.5 with nitric acid. The suspension was treated at90° C. for two hours after which the particles were washed with water toneutrality.

pH Stability Tests

The stability of the various modified particles made in accordance withthe methods of the present invention were investigated in threedifferent ways. Firstly, the modified particles were treated withbuffers of various pH and the leakage was directly monitored in a UVdetector; secondly radioactively labelled ligands were immobilised andthe leakage was detected by release of radioactivity in the supernatant;and thirdly the performance of the particles in HPLC column experimentswas used as an indicator of ligand leakage.

1) direct monitoring of ligand leakage in batch experiment:

The Cibacron Blue modified particles were suspended in 100 mM sodiumcarbonate buffer solutions at different pH-values and shaken overnight.After this time the particles were centrifuged and the supernatant wasexamined for leaking ligands. This was done photometrically at 280 nm.In one experimental series several different buffers were employed for apH stability test of glucose and Cibacron Blue modified zirconia. Thebuffers used were sodium phosphate, sodium carbonate and β-alanine allat concentrations of 100 mM, as well as water titrated with sodiumhydroxide.

2) Detection of ¹⁴ C labelled ligands in batch experiments 100 mg ofmodified zirconia particles were suspended in 2 ml of a 0.1 m sodiumcarbonate buffer and shaken for 24 hours. After this time two samples(each 0.5 ml) were taken and mixed with 4.5 ml scintillation liquid andcounted for 2 min. The particles were resuspended in a new buffer withincreased pH. The pH was increased in steps of 0.5 and the wholeprocedure was repeated up to pH 14.

3) RP chromatographic performance as indicator of ligand leakage.

The octadecyl modified support was packed in a column supplied byBischoff, Leonberg FRG. The column dimensions were 33 mm in length×8 mmID (column volume 1.66 ml). The HPLC equipment used consisted of twoWaters pumps Model 6000A, a Waters gradient former Model 660, aMillipore Waters LC spectrometer Lambda Max Model 481, a Waters DataModule and a DuPont Chartrecorder.

Sample: Aniline, Toluene and Naphthalene (1 mg/ml each)

Solvent: Water+0.1% Trifluoroacetic acid (TFA)

Flowrate: 1 ml/min

Wavelength: 254 nm

The column was exposed to a 0.1M carbonate buffer of pH 9.0 for 1000column volumes with a flow rate of 1.0 ml/min. After each 100 columnvolumes the performance was tested by injecting the test mixture. After1000 column volumes the pH was increased by one. A decrease in retentiontime or in the plate number would have indicated a decrease in ligandcoverage.

No change in the retention time could be observed up to pH 13.

Detection of "Non-Specific" Protein interaction on Carbohydrates andGlymo derivatised Zirconia Sorbents

Four different modified zirconia materials were tested: particlesmodified with glucose, maltose, and also glymo prepared under anhydrousand aqueous conditions. The sorbents were packed in 100×2 mm analyticalcolumns and equilibrated in the chromatographic buffer. Three differentsolvents were used. 10 mM sodium carbonate buffer pH 6.5 with no, 100and 500 mM NaCl added. Three proteins were used as adsorbate: bovineribonuclease A (pI 8.9), bovine carbonic anhydrase (pI 5.9) andovalbumin (pI 4.7). The proteins were run three times each at all saltconcentrations. The total volume was determined with acetone. Theelutions of these proteins were expressed in terms of elution volume ofthe protein divided by the elution volume of the acetone. Since amaterial with 3000A pore size was used, there should be no exclusioneffect and the proteins should elute at the same volume as the acetoneunless interactions between the protein and the particle surface occur.

pH Stability Tests in a Batch Experiment EXAMPLE 11 Using Dye ModifiedZirconia

To determine the chemical stability of the modification the zirconiamodified with Cibacron Blue F3GA was suspended n buffer solutions ofvarious pH and then shaken for 24 hours. After this treatment thesuspension was centrifuged and the supernatant was examined for dyebleeding off the support. When no leakage occurred the whole procedurewas repeated in a buffer adjusted at a pH 0.5 higher than the previous.Under these conditions no leakage occurred at pH 8.0, 8.5, 9.0 and 9.5.At pH=10.0 the supernatant was coloured, indicating that themodification is not stable under these conditions.

The water in the supernatant was evaporated and the solid remaining wasused for an elemental analysis. The material was tested for itsnitrogen, silicon and zirconium content. According to the presumedstructure, the cleavage could occur at three different places:

1. the bead was actually dissolving, which would give positive resultsin the zirconium silicon and nitrogen content,

2. the cleavage occurred between the particle surface and the silicon,giving positive results for the silicon and nitrogen content butnegative results for the zirconium and

3. the cleavage occurred at the sulfur group between the silane and thedye molecule, resulting in very small amounts of both silicon andzirconium.

The actual result of the elemental analysis was 8.8% nitrogen, 1.3%silicon and 0.0047% zirconium, indicating that the cleavage occurred atthe Zr-O-Si bond.

EXAMPLE 12 pH Stability Tests using the Carbohydrate-Dye ModifiedZirconia

A stability test for zirconia was performed using different buffers: aphosphate, a carbonate and a β-alanine buffer. Water titrated with NaOHwas used as a reference. The stability tests were performed with acarbohydrate-Cibacron Blue modified zirconia in a batch experiment. Lossof the modification was monitored at 280 nm. In each case 450 mgzirconia particles were suspended in 5 ml buffer and shaken for 24 hourseach. The experiments were started at pH 9.0 and the pH was increased by1 after each run. The results of this experiment are presented in Table2.

This experiment showed that there were no significant differences instability of this bonded phase in the different buffer solutions,indicating that these ions are not able to displace covalently attachedsilanes from the zirconia surface. The modification in this case showeda high stability, at least up to pH 11.0. The experiments were repeatedseveral times, always with the same result.

The modified zirconia produced with immobilised maltose were stable upto pH 12 as documented in Table 2.

EXAMPLE 13

In this example two materials were compared, one with crosslinking andone without crosslinking. Both materials were zirconia particles made inaccordance with the methods of the present invention and derivatisedwith Cibacron Blue F3GA. A 100 mM phosphate buffer was used. The resultsobtained are set out in Table 3.

These results show clearly that not only the crosslinked material butalso the non-crosslinked particles are remarkably stable. The highstability of these modified zirconia supports were also seen, whenglucose-dye modified particles were suspended in 1M sodium hydroxide andtreated for 24 hours. After washing to neutrality and drying no leakageof the dye could be detected. It is needless to point out that thezirconia particles made in accordance with the present invention show asubstantially higher stability compared to silica particles.

EXAMPLE 14

The results from the "non-specific" protein interaction measurements arepresented in Table 4. The results for the carbohydrate modifiedzirconias, both glucose and maltose, were very similar, so only theresults for the glucose modified particles are listed in this Table.

The three different modified sorbents showed distinctly differentproperties. The Glymo support prepared in water at an acidic pH has apolymeric coating, which is covalently attached to the surface. Thiscoating results in a good coverage of the surface indicated by theprotein elution characteristics of the support. However, this kind ofmodification leads to a thick layer reducing the chromatographicperformance of the support due to increased pore diffusion effects. Boththe carbohydrate modified particles and the support synthesised withGlymo under anhydrous conditions result in a monomeric modification witha controlled thickness of the interactive surface.

From these monomeric modified supports, the particles with thecarbohydrate ligands showed a superior performance over the particlesmodified with Glymo. It is thought that this difference could beexplained by the length of the carbohydrate ligand exceeding that ofGlymo and therefore preventing the protein from reaching the zirconiasurface. There is also a qualitative difference between these twomaterials. While the carbohydrate modified sorbent interacted only withthe most basic protein (Ribonuclease A), the Glymo modified particlesalso adsorbed the more acidic proteins ovalbumin and carbonic anhydrase.This indicates, that there are both Lewis acid and base groups presenton the zirconia surface.

The results of the experiments indicated in the foregoing examples ofthis specification demonstrate an easy method of producingchromatographic sorbent materials having superior chemical stabilitywhen compared to silica based sorbents and having, better physicalcharacteristics than sorbents based on organic polymers.

Modification with Iminodiacetic Acid (IDA)

In the following examples the synthesis of a metal chelate andconconavalin-A modified sorbents and their evaluation are described. Tomodify the zirconia support with IDA the following procedure was used.In a first step the silane is produced. 1 g iminodiacetic acid and 1.503g NaOH are dissolved in 18 ml water and cooled in an ice bath. 1.776 g3-glycidoxypropyltrimethoxy-silane is added dropwise. The solution isstirred and allowed to warm to room temperature and then heated to 60°C. overnight.

To modify the zirconia particles after first subjecting the particles toa hydrothermal treatment, as described above (150° C., 6 hours) toinsure a high and uniform hydroxyl group distribution on the zirconiasurface, a five times excessive amount of the silane solution isadjusted to pH 3.0 with HCL and 1 g of particles is suspended in thissolution. The suspension is heated to 90° C. for three hours giving theresults in FIG. 5.

The particles were washed with 0.1M hydrochloric acid, water and2-propanol and suspended in a solution of coppersulphate to saturate thechelate groups with Cu(II) ions.

EXAMPLE 13 Modification with Concanavalin-A

The modification with a protein is done in two steps. First ahydrothermal treatment, as described above (150° C., 6 hours) wasperformed to insure a high and uniform hydroxyl group distribution onthe zirconia surface. Secondly the support material is modified with3-isothiocyanatopropyl-triethoxysilane to introduce reactive groups ontothe zirconia surface and then the protein is attached via free aminogroups on the protein surface.

To modify the support with the silane, the particles were dried at 180°C. in vacuum. Toluene was dried over sodium metal and freshly distilled.The particles were suspended in the toluene and the silane was added.The amount of silane was calculated for 8 μmol/m² support surface area.A small amount of imidazole was added as a catalyst. The suspension wassonicated for five minutes to remove air trapped inside the pores. Themixture was treated under reflux for 24 hours and then washed withtoluene, 2-propanol and water.

To attach the protein, the modified particles were suspended in acetatebuffer pH 6.5 and 10 mM MnCl₂ and CaCl₂ were added to maintain thebiological activity of Con-A. The suspension was treated at roomtemperature for 48 hours and washed with the same buffer. The particleswere never dried. The results are shown in FIG. 6.

To block remaining NCS-groups the particles were treated with a solutionof ethanolamine pH 7.0 overnight.

EXAMPLE 14 Batch Adsorption Experiments with Metal Chelate ModifiedZirconia

1 g of 3M zirconia was suspended in 20 ml of 20 mM phosphate buffer pH7.0 with 0.2M NaCl added. Due to the lower specific surface area of thePDZ particles 2 g were used for the adsorption experiments with thissupport material. Horse heart myoglobin was dissolved in the same bufferused for the suspension at a concentration of 1 mg/ml and addedsuccessively to metal chelate sorbents. During the whole experiment thetemperature of the suspension was kept at 7° C. The rate of adsorptionwas monitored at280 mn and recorded until equilibrium was reached. Theequilibrium concentrations were used to plot the adsorption isothermwhich was evaluated using three different linearisation approaches(double reciprocal plot, semi reciprocal plot and Scatchard plot 5-10!.

EXAMPLE 15 Batch Adsorption Experiments with Concanavalin-A ModifiedZirconia

For the adsorption of horseradish peroxidase on Concanavalin-A modifiedzirconia a 20 mM phosphate buffer with 0.2M NaCl added was adjusted topH 6.5. The buffer contained 1 mM of each CaCl₂, MnCl₂ and MgCl₂ tosustain the biological activity of Concanavalin-A. The buffer wasfiltered prior to use to remove undissolved Mn- or Ca-phosphateprecipitation. As before either 1 g of 3M zirconia or 2 g of PDZzirconia was used in each experiment. The particles were suspended in 25ml of the buffer and the suspension was thermostatted at 7° C.Horseradish peroxidase was dissolved in the described buffer at aconcentration of 1 mg/ml. To examine whether the binding was due tospecific interaction the adsorbate was eluted after the recording of theadsorption isotherm was completed using methyl-D-mannopyranoside and theadsorption step was repeated.

To evaluate the adsorption data the same approach as in case of themetal chelate modified supports was employed. In both cases theadsorption coefficient of the protein under the chosen conditions wasdetermined by recording a calibration curve without the presence ofsorbent material. The results for Qm and Kd are listed in Table 5. Theutilisation of the 3M zirconia for the Con-A affinity adsorptionappeared to be not practical due to a significant reduction of the poresize by the ligand and a resulting very restricted pore diffusion of theadsorbate. The result of the pore diffusion appears when the adsorptionkinetics for the adsorption of myoglobin on IDA-zirconia is comparedwith the adsorption of peroxidase onto Con-A modified particles. Anincrease in temperature to 25° C. in order to increase the diffusionkinetics did not improve the results in a satisfying way, so only theresults obtained with the PDt zirconia at 7° C. are presented. Theresults for Qm and Kd are listed in Table; 6.

The good concordance between the first adsorption and the consecutiveexperiment after specific elution indicates that the binding ofperoxidase to the sorbent is due to specific interactions between thecarbohydrate binding site of Concanavalin-A and the glyco-part of theperoxidase molecule and that the elution step is complete to retain theoriginal capacity.

The results obtained in the foregoing examples of this specificationshow clearly that the modification chemistry for various sorbents canalso be applied to synthesise affinity supports. The zirconia particlesof the present invention can be surface modified in a variety of ways inaccordance with diverse chemical separation applications that thesurface modified particles are to be used in.

EXAMPLE 16

Desirable properties of support material for immobilisation of enzymesare: hydrophilic surface characteristics, good packing properties, highsurface area, permeability and mechanical stability.

Two inorganic, porous sorbent materials were used as a matrix to attachthe proteases. The main focus was put on a porous zirconia. Due to thehigh density of zirconia, these particles offer ideal characteristicsfor use in bioseparators. They exhibit a superior settling rate inclosed systems and allow higher flowrates in continuous reactors.

For the immobilisation of the protease the particles of zirconia wereactivated with 3-isothiocyanatopropyl-triethoxysilane as described inExample 13.

Coupling of the enzyme to the activated carrier. 200 mg activatedsupport were suspended in 10 ml buffer solution containing 11 mgprotease. The suspension was shaken headover at room temperature for 24hours. After the coupling procedure, the suspension was filtered and thesupernatant preserved. The immobilised enzyme was washed with 0.5M NaCland the remaining NCS-groups were blocked with ethanolamine.

The coupling procedure were modified in order to accommodate thespecific requirements of each enzyme. For pepsin lower pH values wereused since pepsin is rapidly and irreversibly denaturated at alkaline pHvalues, but is stable between pH 5 and 5.5; the presence of calciumchloride in the coupling mixture for trypsin improves the specificactivity of the immobilised trypsin by reducing the autodigestion. Theapplication of buffers which contain amino groups have been avoidedduring the coupling process to avoid blocking of the NCS groups by thesebuffers.

The following coupling conditions were applied for the differentproteases:

Trypsin:

a) 0.02M CaCl₂ -solution, pH 7.0

b) 100 mM HEPES buffer with 0.02M CaCl₂, pH 8.0

c) 199 mM Clark and Lubs solution (according to Elliot et al) with 0.02MCaCl₂, pH 9.0

Chymotryspin:

a) 0.02M CaCl₂ solution, pH 7.0

b) 100 mM HEPES, pH 8.0

Papain:

a) water, pH 7.0

b) 100 mM HEPES buffer, pH 8.0

c) 100 mM Clark and Lubs solution, pH 9.0 (only with zirconia)

Pepsin:

a) 100 mM Citrate buffer, pH 5.0

b) 100 mM Citrate buffer, pH 5.5

c) 100 mM Citrate buffer, pH 6.0

d) 100 mM Acetate buffer, pH 4.5

e) 100 mM Acetate buffer, pH 5.6

a), b) and c) were performed with silica only.

After the coupling and blocking of remaining NCS groups the enzymederivatives were washed extensively with buffer and stored at roomtemperature in the following buffers:

Trypsin: in 100 mM Tris/HCl, 20 mM CaCl₂, pH 8.0

Chymotrypsin: in 100 mM Tris/HCl, pH 8.0

Papain: in 100 mM Acetate buffer, pH 5.0

Pepsin: in 50 mM Acetate buffer, pH 4.0

EXAMPLE 17

Humic substances appear in all open water sources, and their removal isan important task to improve the water quality. Although humic acid isnot toxic per se it Gas a distinct brownish colour, making the waterless attractive to the consumer. Because of the huge volumes involved,an effective water purification method has to be efficient, fast andinexpensive. A stirred tank or fluidised bed adsorption setup withdense, high capacity particles is preferred to the more costlyalternatives, e.g. packed bed purification systems, because of thescale-up requirements (megalitres per hour requirements are oftenencountered in water process facilities) and the associated processeconomics. The particles act as ion exchangers, typically anionicexchangers.

A weak anionic exchanger(4-amino-4',4"-bisdimethylamino-triphenylcarbinol, 4-amino malachitegreen) was synthesised by condensing 1 part p-nitrobenzaldehyde with 2parts N,N'-dimethylaniline. The nitrogroup was reduced to form an aminegroup, which also reduced the carbinol group. The carbinol group wasreintroduced in a third step to form the target compound.

Zirconia was hydrothermally treated as described before and modifiedwith 3-isothiocyanotopropyltriethoxysilane to introduce NCS-functionalgroups to the zirconia surface, which are able to react with the aminegroup of the 4-amino malachite green molecule.

For the preparation of the strong anionic exchangers, the zirconia (orsilica) particles were modified with a polystyrene-based coating. Thezirconia particles were hydrothermally treated as described previouslyand modified with 3-aminopropyltriethoxy-silane. Styrene was polymerisedusing anionic polymerisation, initiated with sodium naphthalene toachieve a narrow molecular weight range. The polymer waschloromethylated and coupled to the amino-modified zirconia or silica.An excessive amount of chloromethylated polystyrene was added ensuringthat only a small portion of the chloromethyl groups of a givenpolystyrene chain had the chance to react with the activated zirconia,leaving the majority of the chloromethyl groups available for thegeneration of ion exchange groups as well as resulting in a tentacletype modification. The unreacted chloromethyl groups were derivatisedwith either trimethylamine or triethylamine resulting in a zirconiaadsorbent chemically coated with polystyrene-trimethylammonium chlorideor polystyrene-triethylammonium chloride groups.

To determine the effectiveness of using surface modified zirconia inremoving humic acid from a river, water sample obtained from a river inthe wine growing district of South Australia was obtained and differentamounts of the modified zirconia adsorbents were suspended in 50 ml ofthe water sample under controlled temperature conditions and theadsorption process monitored continuously from the change in opticalabsorbance at 254 nm.

It was found that the humic acid substances in Barossa Valley waterconsist of a variety of compounds with different affinities for ionexchangers of different strength. Three different surface modificationprocedures resulted in adsorbents which exhibit different ion exchangecapacities. A malachite green modification can be considered a weakanion exchanger while the triethyl- andtrimethyl-phenylammonium-chloride modifications are strong ionexchangers. The trimethyl modification resulted in an even stronger ionexchanger than the triethyl modification. The various strengths of theion exchangers dominate the way the adsorbents interact with the humicsubstances in the water. Besides the maximum capacity, the adsorptionkinetics are also a very important feature in the adsorption processbecause they determine the throughput or the efficiency of the system.The best performances were observed when the malachite green and thetrimethylphenylammonium modifications were used, whilst thetriethylphenylammonium modified SAX particles showed significantlyslower kinetics.

Due to the higher density of zirconia however, these particles shouldhave a distinct advantage in terms of their settling rate when they areemployed in large scale expanded bed processes. Faster settling ratesmean faster separation times between the liquid and the solid phase andan increase in efficiency due to a reduced cycle time. Another importantAdvantage of surface modified zirconia adsorbents is the high chemicalstability over a wide range of pH conditions, thus offering a greatervariety of elution and regeneration possibilities.

Advantages and Industrial Applicability

The uniform distribution of pores in the porous zirconia particle renderporous zirconia of the present invention particularly useful inapplications relating to the separation of chemicals and biochemicals aswell as for use as supports for catalysts and catalyst compositions.

The porous zirconia of the present invention is chemically stable andcan be used in alkaline media in which porous silica fails. The porouszirconia has good strength and is of high density when compared toporous silica and organic polymers. The porous zirconia of the presentinvention may be used in the purification of high value chemicals,polymers and high molecular weight biochemicals using packed orfluidised beds of porous zirconia particles. The porous zirconia of thepresent invention may also be used for the analysis of high molecularweight polymers and biochemicals by chromatographic techniques usingimmobilised low molecular weight ligands bound to the surface of theporous zirconia. The porous zirconia of the present invention may alsobe used for the chromatographic analysis of chemicals and biochemicalsusing highly specific, bound, high molecular weight ligands or the like.other applications for the use of porous zirconia of the presentinvention include bio-sensors which may be used in on-line sensors forprocess and environmental control, as supports for bio-catalysts and assupports for conventional catalysts.

The porous zirconia of the present invention may also be used toseparate contaminants. Such separation applications include the recoveryof product from reaction mixtures. These include the recovery in thedownstream processing of fermentation broths or cell cultures or as analternative to ultrafiltration. The porous zirconia may be used as asorbent for separation of micellar mixtures without liquid/liquidextractions. The porous zirconia may also be used for high resolutionremoval of toxins or contaminants from process streams or recovery ofhigh value inorganic materials such as the rarer metals or the like. Theporous zirconia may also be used for the removal of liquid aerosols fromgas streams with, or without, recovery of the liquid phase.

The porous zirconia of the present invention which has been used inchromatographic, separative and catalysis applications and which hasbeen spent may be readily regenerable by the burning of any organicmatter out of the porous zirconia. This is particularly advantageouswhere the porous zirconia is used to extract organic molecules fromprocess streams.

                  TABLE 1    ______________________________________                 amount of phosphate                               free surface    Material     adsorbed  mg! area  %!    ______________________________________    3M not modified                 1.53          100    3M Cibacron Blue mod.                 0.39          25.5    3M Glucose modified                 0.67          43.8    PDZ not modified                 0.158         100    PDZ Cibacron Blue mod.                 0.036         22.8    PDZ C.sub.18 modified                 0.023         14.5    ______________________________________

                  TABLE 2    ______________________________________    Buffer/pH             9.0       10.0   11.0    12.0 13.0    ______________________________________    phosphate             0.26      0.25   0.25    0.31 0.46    β-alanine             0.22      0.25   0.33    0.38 0.56    carbonate             0.25      0.32   0.34    0.36 0.56    NaOH     0.31      0.26   0.28    0.46 0.59    ______________________________________

                  TABLE 3    ______________________________________    Support/pH             9.0     10.0    11.0  12.0  13.0  14.0    ______________________________________    non crossl.             0.027   0.031   0.031 0.037 0.077 0.081    crosslinked             0.025   0.032   0.029 0.029 0.054 0.076    ______________________________________

                  TABLE 4    ______________________________________    Protein      no salt   100 mM NaCl                                      500 mM NaCl    ______________________________________    a   Ovalbumin    0.97      1.05     1.18        Carbonic anhydrase                     1.07      1.07     1.05        Ribonuclease A                     not eluted                               2.01     1.04    b   Ovalbumin    not eluted                               1.09     0.99        Carbonic anhydrase                     not eluted                               not eluted                                        1.32        Ribonuclease A                     not eluted                               1.74     1.20    c   Ovalbumin    0.96      1.02     1.07        Carbonic anhydrase                     0.99      1.01     1.04        Ribonuclease A                     not eluted                               1.27     1.13    ______________________________________     Table 4: Protein interaction on different hydrophilic modified zirconia     supports:     a Glucose modified particles     b Glymo modified under anhydrous conditions     c Glymo modified under acidic aqueous conditions     The elution of the proteins is expressed in elution volume of the protein     divided by the elution volume of acetone

                  TABLE 5    ______________________________________           double rec. plot                     semi rec. plot                                 Scatchard plot    ______________________________________    3M zirconia    all data used             q.sub.m  = 3.68 · 10.sup.-2                         q.sub.m  = 3.19 · 10.sup.-5                                     q.sub.m  = 3.58 · 10.sup.-5             K.sub.d  = 5.09 · 10.sup.-4                         K.sub.d  = 2.83 · 10.sup.-7                                     K.sub.d  = 3.61 · 10.sup.-7    5 smallest             q.sub.m  = 3.53 · 10.sup.-5                         q.sub.m  = 3.18 · 10.sup.-5                                     q.sub.m  = 3.34 · 10.sup.-5    conc.    neglected             K.sub.d  = 3.27 · 10.sup.-7                         K.sub.d  = 2.49 · 10.sup.-7                                     K.sub.d  = 2.89 · 10.sup.-7    PDZ zirconia    all data used             q.sub.m  = 4.46 · 10.sup.-6                         q.sub.m  = 7.67 · 10.sup.-6                                     q.sub.m  = 6.84 · 10.sup.-6             K.sub.d  = 4.65 · 10.sup.-8                         K.sub.d  = 1.76 · 10.sup.-7                                     K.sub.d  = 1.16 · 10.sup.-7    spurious data             q.sub.m  = 6.04 · 10.sup.-6                         q.sub.m  = 7.75 · 10.sup.-6                                     q.sub.m  = 7.00 · 10.sup.-6    points   K.sub.d  = 1.12 · 10.sup.-7                         K.sub.d  = 2.12 · 10.sup.-7                                     K.sub.d  = 1.55 · 10.sup.-7    neglected    ______________________________________

                  TABLE 6    ______________________________________           double rec. plot                     semi rec. plot                                 Scatchard plot    ______________________________________    PDZ zirconia modified with Con-A: first adsorption experiment    all data used             q.sub.m  = 1.76 · 10.sup.-5                         q.sub.m  = 5.65 · 10.sup.-6                                     q.sub.m  = 5.54 · 10.sup.-6             K.sub.d  = 5.16 · 10.sup.-6                         K.sub.d  = 1.48 · 10.sup.-6                                     K.sub.d  = 1.42 · 10.sup.-6    smallest conc.             q.sub.m  = 4.66 · 10.sup.-6                         q.sub.m  = 5.59 · 10.sup.-6                                     q.sub.m  = 5.39 · 10.sup.-6    neglected             K.sub.d  = 1.08 · 10.sup.-6                         K.sub.d  = 1.43 · 10.sup.-6                                     K.sub.d  = 1.33 · 10.sup.-6    second adsorption after elution with α-methylmannose    all data used             q.sub.m  = 1.32 · 10.sup.-5                         q.sub.m  = 4.92 · 10.sup.-6                                     q.sub.m  = 6.31 · 10.sup.-6             K.sub.d  = 6.35 · 10.sup.-6                         K.sub.d  = 1.97 · 10.sup.-6                                     K.sub.d  = 2.70 · 10.sup.-6    ______________________________________

We claim:
 1. A method for the production of porous zirconia comprisingthe following steps in sequence:(a) heating a zirconia-silicacomposition to provide particles of said composition in a substantiallyhomogeneous liquid melt form; (b) quenching said particles to effectspinodal decomposition of the liquid melt to provide quenched particlescomprising a silica-rich phase and a zirconia-rich phase, wherein saidzirconia-rich phase comprises zirconia substantially in a tetragonalform; (c) annealing said quenched particles to transform the tetragonalform of the zirconia rich phase to the monoclinic form on cooling so asto provide annealed particles comprising a continuous monocliniczirconia-rich phase and a continuous silica-rich phase; (d) leachingsaid silica-rich phase from the annealed particles to provide porousmonoclinic zirconia particles each of said particles comprising a threedimensionally continuous interpenetrating network of interconnectedpores, said pores being of substantially constant diameter throughouttheir length.
 2. A method according to claim 1 in which a third phase isformed in step (c).
 3. A method according to claim 2 in which the thirdphase is zircon.
 4. A method according to claim 3 in which the zircon isnot leached away from and remains in the zirconia phase after leachingwhen the silica phase is leached away.
 5. A method according to claim 4in which the porous zirconia particles comprise zirconia and zircon inwhich the zircon is intimately incorporated into the zirconia.
 6. Amethod according to claim 1 in which the zirconia-silica composition isa substantially uniform composition or provides a liquid melt of asubstantially uniform composition.
 7. A method according to claim 1 inwhich the zirconia-silica composition is an admixture of zirconia orzirconia-containing material and a silica or silica-containing materialor a compound containing both zirconia and silica or a composition,mixture or compound which decomposes to provide a substantiallyhomogeneous liquid melt of zirconia and silica.
 8. A method according toclaim 7 in which the molar ratio of zirconia to silica is about 1:1. 9.A method according to claim 1 in which the percentage of zirconia tosilica in the quenched particles is from about 100-1% molar zirconia to0-99% molar silica.
 10. A method according to claim 1 in which thezirconia-silica composition is zircon.
 11. A method according to claim 1in which the zirconia-silica composition is in the form of a powder orparticles.
 12. A method according to claim 1 in which the pore size of asingle zirconia particle is substantially constant and there is adistribution of varying pore sizes between the particles, such as adistribution of 0.01 to 0.2 μm for pore size in a distribution ofparticle size range of 40 to 80 μm.
 13. A method according to claim 1 inwhich the zirconia-silica composition forms the liquid melt attemperatures in excess of about 2400° C.
 14. A method according to claim1 in which heating of the zirconia-silica composition occurs in a plasmaarc torch or similar.
 15. A method according to claim 1 in which theparticles of zircon heated in the plasma arc torch have a particle sizein the range of from 5 to 100 μm.
 16. A method according to claim 1 inwhich the zircon particles are elongated and on heating the particlesfirst dissociate and then melt to form a substantially homogeneousliquid melt.
 17. A method according to claim 1 in which thezirconia-silica composition is heated by flame spraying using anoxyacetylene flame.
 18. A method according to claim 17 in which thezirconia-silica particles are heated by flame spraying with anoxyacetylene flame and the particle sizes so treated are in the range offrom 3 to 15 μm.
 19. A method according to claim 1 in which theparticles of the substantially homogeneous liquid melt are quenched at acooling rate sufficient to prevent nucleation and growth of zirconiaspherulites and to allow spinodal decomposition of the liquid melt intoa substantially fine micro-structure of zirconia-rich and silica-richphases, optionally containing a further phase.
 20. A method according toclaim 1 in which the quenched particles have wave lengths of about 100 Åand have uniform periodicity and three dimensional continuity.
 21. Amethod according to claim 1 in which homogeneous liquid melt is quenchedin a liquid.
 22. A method according to claim 1 in which quenchingprovides a cooling rate in the range of about 10⁵ to 10⁷ ° C. sec⁻¹. 23.A method according to claim 1 in which the quenched particles compriseboth a zirconia-rich and silica-rich phase in which the zirconia-richphase is substantially in tetragonal form.
 24. A method according toclaim 1 further comprising additional leaching of the silica-rich phaseso that the zirconia-rich phase will transform from tetragonal to stablemonoclinic form.
 25. A method according to claim 1 in which theannealing takes place below a temperature at which there issubstantially no recombination of zirconia and silica.
 26. A methodaccording to claim 25 in which some recombination of zirconia and silicaoccurs to form zircon to enhance the strength of the porous particlesproduced by leaching the silica therefrom, said zircon being optionallyincorporated into the zirconia phase.
 27. A method according to claim 1in which annealing takes place at a temperature in the range from about1000° C. to 1400° C., over a period 5 hours.
 28. The method of claim 1in which the transformation temperature of dissociated zircon quenchedand subsequently annealed is about 720° C.
 29. A method according toclaim 1 in which the transformation of the tetragonal form to themonoclinic form does not lead to substantial shattering of the annealedparticles on cooling.
 30. A method according to claim 1 in which alkalior hydrofluoric acid are used to leach the silica-rich phase.
 31. Themethod of claim 30 in which the annealed particles are leached withalkali, at a temperature of about 160° C.
 32. A porous zirconia particleor composition made accordance with the method of claim
 1. 33. A porouszirconia particle produced by the method recited in claim
 5. 34. Aporous zirconia particle produced by the method recited in claim
 6. 35.A porous zirconia particle produced by the method recited in claim 24.36. A method for the production of porous tetragonal zirconia comprisingthe following steps in sequence:(a) heating a zirconia-silicacomposition to provide particles of said composition in the form of asubstantially homogeneous liquid melt; (b) quenching said particles suchthat spinodal decomposition of the liquid melt occurs to providequenched particles comprising a silica-rich phase and a zirconia-richphase; (c) annealing said quenched particles to coarsen thezirconia-rich phase such that the desired pore size can be obtained instep (d); (d) leaching said silica-rich phase from the annealedparticles to provide porous tetragonal zirconia particles comprised ofthree dimensionally continuous interpenetrating networks of pores, saidpores being of substantially constant diameter throughout their length.37. The method of claim 36 in which the tetragonal zirconia isstabilised by the addition of dopants of rare earth oxides or othermetal oxides, or combinations thereof.
 38. The method of claim 36 inwhich the zirconia-silica composition further comprises a dopantintimately incorporated into the composition.
 39. The method of claim 38in which the dopant is yttria.
 40. The method of claim 36 in which theparticles in said step (b) comprise a third phase of a dopant of ytrria,the yttria being incorporated into the zirconia-rich phase.
 41. Themethod of claim 40 in which the dopant is calcia, or magnesia.
 42. Aporous zirconia particle or composition made accordance with the methodof claim
 36. 43. A porous zirconia particle produced by the methodrecited in claim
 37. 44. A method for the production of porous cubiczirconia comprising the following steps in sequence:(a) heating azirconia-silica composition to provide particles of said composition inthe form of a substantially homogeneous liquid melt; (b) quenching saidparticles to effect spinodal decomposition of the liquid melt to providequenched particles comprising a silica-rich phase and a zirconia-richphase; (c) annealing said quenched particles to coarsen thezirconia-rich phase such that desired pore size can be obtained in step(d); (d) leaching said silica-rich phase from the annealed particles toprovide porous cubic zirconia particles comprised of three dimensionallycontinuous interpenetrating networks of interconnected pores, said poresbeing of substantially constant diameter throughout their length.
 45. Amethod according to claim 44 in which the particles in step a further instep a comprise a dopant.
 46. A method according to claim 44 in whichthe zirconia-silica composition further comprises a dopant intimatelyincorporated into the zirconia-silica composition.
 47. The method ofclaim 44 in which the particles in said step (b) comprise a third phaseof a dopant.
 48. A porous zirconia particle produced by the methodrecited in claim
 44. 49. A porous zirconia particle produced by themethod recited in claim 45.